Share page-check helpers in tests/page_check_common.py

MODES, TEST_FILL, FAKE_BRIDGE, the stub loader, the page opener and the no-AI case list now live in one place; sample_data and push move to tests/conftest.py.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/page_check_ai_desktop/test_ai_desktop.py

@@ -25,17 +25,9 @@ from test_ai_cost import (  # noqa: E402,F401
     test_spend_tiles_sum_the_priced_runs,
 )
 
-sys.path.insert(0, str(Path(__file__).resolve().parents[1] / "page_check"))
-from test_contract import (  # noqa: E402,F401
-    test_dark_toggle_flips_theme,
-    test_donut_and_legend_match_batch_states,
-    test_kpi_counts_match_batch_states,
-    test_log_lines_show_their_type,
-    test_no_bot_page_runs_on_sample_data,
-    test_plain_browser_first_open_shows_sample_data,
-    test_report_search_filters_rows_and_shows_bucket_pill,
-    test_step_list_follows_step_label,
-)
+from page_check_common import no_ai_cases  # noqa: E402
+
+globals().update(no_ai_cases())
 
 TAMPER = """async (path) => {
   const reply = new Promise(done => { const base = window.__pyMsg;